Page 108 - Geology and Geochemistry of Oil and Gas

80 WATER

Gravity water can move as a result of gravity force, obeying the Darcy’s law; it
transmits the hydrostatic pressure. Surface tension affects the water not as strongly
as the gravity force, and has only slight effect on permeability. With increasing role
of interfacial tension, water can be classified as ‘‘capillary.’’ Thus, gravity can be in
equilibrium with interfacial tension. Capillary water can also transmit hydrostatic
pressure, but very slowly (within the lower limit of applicability of Darcy’s law). A
further increase in the role of interfacial tension results in the appearance of loosely
bonded water (film water, physically bonded water).
The properties of film water are quite different from that of free water. It moves
according to the law of diffusion toward equalization of the film concentration on
the grain surfaces. Firmly bonded water (adsorption water) is retained in place by
surface tension (41,100 MPa), but is easily removed as steam with increasing
temperature. Upon thermolysis, the first peak occurs at a temperature above 100 1C.
Some authors (see e.g., Beletskaya, 1990; Simonenko, 1988) call this firmly bonded
water a ‘‘sub-melted’’ water layer, 0.01–0.006 mm thick. This layer can move
somewhat easier than the physically bonded water. The molecule orderliness,
viscosity, and shear strength in such ‘‘sub-melted’’ water are less that in the
irreducible water (Tsarev, 1978).
When a hydrocarbon accumulation is formed, hydrocarbons push the free water
out of the accumulation, whereas the capillary water is retained. After the oil and gas
are produced, the equilibrium in the reservoir changes. The ‘‘sub-melted’’ and some
capillary water is produced and may account for between a fraction of one percent
and a few dozen percents of the production. The content of such water depends on
surface phenomena, and that is why this content is greater in beds with greater active
surface. In a clay–sand–carbonates series, the content of such water decreases in the
reservoirs as well as in the produced fluids. Gas produced from carbonate reservoirs
may contain as low as 1% of water.
There are transitions between the aforementioned types of water. Sometimes more
detailed subdivisions are used, such as mono- and poly-molecular layers (i.e., the
number of molecules in a layer), in the water film surrounding the grains. If bonded
water becomes free, its general properties change: density increases, and the solution
capacity decreases. Thus, the released bonded water is fresher and aggressive.
Chemically bonded water is a constituent part of minerals. It includes the crystal-
hydrate (zeolite) and constitution waters. The amount of crystallization water in
minerals is not constant and may be removed by heating to 450–500 1C. Interlayer
montmorillonite water is the crystallization water. A mineral does not necessarily
decompose after the removal of crystallization water. It may only change its form
(e.g., gypsum-anhydrite; montmorillonite-illite). The removal of the constitution
water, on the other hand, results in the destruction of mineral. It occurs when
temperature exceeds about 500 1C.

4.3.1. Water drive systems

The studies of water drive systems in the oil (gas) fields and basins followed the
studies of hydraulic systems in artesian basins. The term ‘‘oil/gas basin’’ was adopted